What indicates a positive result in a PPD skin test?

A positive result in a PPD (Purified Protein Derivative) skin test is indicated by the presence of induration at the injection site, specifically, a firm, raised area that develops as an immune response to the tuberculosis antigens in the PPD. This induration is measured in millimeters and is a sign that the person has been exposed to the bacterium Mycobacterium tuberculosis or has previously been sensitized to it.

Induration distinguishes a positive test from simple redness or swelling. Redness alone does not indicate a positive result; it might occur without significant immune reaction. Additionally, fever is not an expected response to the PPD test and would not indicate a positive result. Visible swelling can occur but must be combined with the specific measurement of induration to signify a positive reaction. Therefore, induration is the key factor that healthcare providers look for when interpreting the results of a PPD skin test.